WordNet (r) 3.0 (2006):

sixth sense
    n 1: grasping the inner nature of things intuitively [syn:
         insight, sixth sense]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "sixth sense":
   ESP, a priori knowledge, anticipation, buddhi, chromesthesia,
   clairsentience, clairvoyance, color hearing, direct apprehension,
   extrasensory perception, five senses, foresight, hearing,
   immediate cognition, insight, inspiration, intuition, intuitionism,
   intuitive reason, intuitiveness, intuitivism,
   knowledge without thought, lucidity, phonism, photism,
   precognition, premonition, psychometry, receptor, satori,
   second sight, second-sightedness, sense organ, senses, sensillum,
   sensorium, sensory organ, sight, smell, subconscious knowledge,
   subconscious perception, synesthesia, taste, touch,
   unmediated perception